In an optical communications network, optical data signals transmitted by a transmitter unit are received by a receiver unit. The receiver unit comprises a photodetector to collect optical electromagnetic radiation including the optical data signals and convert the collected optical data signals to electrical data signals for processing. One known circuit for collecting the optical data signals and converting the optical data signals to the electrical data signals comprises a reverse biased APD coupled between a positive supply rail and a transimpedance amplifier.
In order for the APD to reside in an operating state within a range of operating states so as to be capable of generating a sufficiently large photocurrent in response to optical electromagnetic radiation incident on the APD, a reverse bias voltage, for example about 70V, close to the breakdown voltage of the APD, is applied across terminals of the APD. To ensure optimum operation of the APD over temperature variations of the APD, a control loop is employed to maintain the photocurrent at an optimum level, since changes in temperature affect an optimum bias voltage of the APD. In addition to the effects of temperature, the manufacturing variances associated with APDs result in variations in the photocurrent. There is therefore a clear need to ensure that the photocurrent flowing through the APD is optimum.
To measure the photocurrent, it is known to employ a current mirror. Two candidate connections exist for coupling the current mirror to the APD. A first candidate connection is between the photodiode and a transimpedance amplifier, but this connection results in the introduction of unwanted noise. A second candidate connection is between the positive supply rail and the APD. However, in order to reverse bias the APD, the positive supply is set to, for example, about 70V, resulting in about 70V being applied across two transistor terminals of the current mirror, i.e. across a sensing input terminal and a monitoring output terminal. The current mirror with the second candidate connection is therefore difficult to implement.